Explain Class Mammalia in animal kingdom?

Class Mammalia: Mammals are distinguished by their hair and their mammary glands. Mammary glands, after which this group of vertebrates is named, produce milk to nourish their young during development. Mammals are endothermic, and hair functions as insulation that helps to maintain a uniform body temperature.

Most mammals have offspring that develop within a uterus. The embryo's development within the uterus is supported by a placenta, which connects it to the mother's bloodstream. The connection supplies food and water, allows for the exchange of gases, and serves to remove waste. Because this connection is so important, the mammals in this group are referred to as the placental mammals.

Only three species of mammals reproduce by laying eggs. They all live in Australia and New Guinea, and are known as the duckbilled platypus and the spiny anteater. These egg-laying mammals belong to a group called the monotremes.

A third group of mammals is the marsupials. Marsupials give birth to very young embryos (in some cases only 8 days following fertilization), which then complete their development within a pouch called the marsupium, on the mother. Mammals are classified into three major groups according to whether they are monotremes, marsupials, or placental.

Mammals are also characterized by their teeth, which have become modified for specialized functions. Incisor and canine teeth cut, pierce, and tear flesh, or are used for gnawing. Premolars and molars are used for crunching, crushing, and grinding, depending on the animal. Even more dramatic modifications of teeth are seen in tusks, which have various uses among the different species.

The circulatory system in mammals is advanced, with a four-chambered heart that drives two circuits, or loops. One loop delivers oxygen-depleted blood to the lungs for oxygenation, and the other loop pumps oxygen-rich blood to the entire body.

Mammal ancestors evolved from the reptiles approximately 220 million years ago, as evidenced by mammal-like reptile fossils from the Permian Period. These fossils indicate that mammals were contemporaries of the dinosaurs. The mammal ancestors were of smaller stature than the dinosaurs, however. Whatever events befell the primitive world and caused the eventual demise and ultimate extinction of the dinosaurs, somehow did not wipe out all of the mammals. Surviving mammals went on to proliferate and evolve, filling the void of ecological niches left by the extinct dinosaurs. Coincident with this flush of mammal evolution, the flowering plants replaced the great expanses of gymnosperms, which also waned during the same period.

There are 19 Orders of mammals recognized today. Listed below are 12 of the major Orders of mammals.

Artiodactyla - sheep, pigs, cattle, deer, giraffes

Carnivora - dogs, wolves, weasels, otters, seals, walruses, bears, cats,

Cetacea - whales, porpoises, dolphins

Chiroptera - bats

Edentata - anteaters, armadillos, sloths

Insectivora - hedgehogs, moles, shrews

Lagomorpha - rabbits, hares

Perissodactyla - horses, zebras, rhinoceroses

Primates - humans, monkeys, apes, lemurs

Proboscidea - elephants

Sirenia - manatees

Rodentia - rats, mice, squirrels, beavers, porcupines